Studies of the physical and genetic organization of the SV40 genome are continuing. Our principal approach involves the use of mutants with deletions in various regions of the viral DNA to analyze various aspects of SV40 gene expression, e.g., transcription, processing and translation, during lytic growth and in SV40 transformed cells. The SV40 genomic DNA will be used to propagate and integrate exogenous DNA segments in mammalian cells. Various DNA segments from simple eukaryotes (e.g., yeast) will be incorporated at various locations in the viral DNA to determine the optimal location and mode of joining for maximal and proper expression.